Exactly. I never said they underfuel, that would be breaking the FARs. The aircraft have to be able to meet TERPS climb gradient requirements for the particular departure they're doing, and if it comes down to booting pax or cargo or having less fuel (but still in legal limits), they'll do that, such as the SLC-CDG flight; sometimes in the summer, they lurch off the runway and climb as slow as my flight school trainer aircraft, stop in DTW for fuel, and then continue on. It doesn't even have to be a heavy. The CR2, with 50 pax, an FA, and two pilots plus a jumpseater, can be fairly close to MTOW with the tanks topped off. Weather, engine performance, and many other factors also come into play. You can input an anticipated fuel burn during taxi into the FMS, and even taxi around with the engines spooled up and the spoilers/brakes applied, to help that out. But you're maybe getting a couple hundred pounds out of that. Yet again, on an AF A340, we took off out of DTW, climbed very slow, and leveled off at 3000', and continued for about 10 minutes before proceeding to the low FL300s. We eventually were able to get on up to 350 or 370 by the time I drifted off to sleep.

So yes, I may not have an original copy of a DL manual stipulating exactly how much or little fuel a particular aircraft will carry, but you're kidding yourself if you think every airplane takes off with full fuel. And even those that do (such as a 10 hour TATL) will be running fairly dry when they're landing, and may have the required reserves (enough to fly to the alternate, then 45 minutes after at cruise). Which, on a related note, is one reason CVG was an alternate for DTW. Distance requirements for Part 121 operators really varies, but may be as little as 45 minutes away, but could be significantly greater (in the case of a four-engined airplane) even several hours.